Satellizer emerges as a primary antagonist in the Alicization narrative, later unveiled as the real-world identity of Gabriel Miller, an American executive involved in the Underworld virtual universe project. Gabriel Miller harbors a disturbing psychological compulsion to collect human souls as personal fascinations and trophies. This fixation stems from childhood trauma surrounding Alicia Klingerman's death; convinced he absorbed her soul, he developed an unshakable belief in his ability to capture souls.

Before his Underworld involvement, Gabriel led the murderous guild Laughing Coffin during Sword Art Online's death game. He nurtured a specific vendetta against Kirito from their SAO confrontations. Gabriel's infiltration of Underworld intertwines his soul obsession and revenge motive against Kirito.

Using Soul Translator technology, Gabriel invades Underworld as the avatar "Subtilizer" during the Human Empire's War of Underworld against the Dark Territory. His avatar mirrors his real-world appearance. Operating under the alias Satellizer, he commands the Dark Territory's assault as a general. His objective transcends military victory: he targets Underworld inhabitants' high-quality Fluctlights (souls), prioritizing Alice Zuberg for her perceived strength and purity.

Satellizer/Gabriel wields formidable power in Underworld, leveraging ruthless tactics and deep system knowledge. His abilities enable direct attacks to consume others' Fluctlights, reflecting his real-world pathology. He exhibits particular cruelty toward Kirito—initially catatonic—exploiting this vulnerability. His actions ignite critical conflict in the war's climax.

Satellizer faces defeat within Underworld. Despite his power, Kirito's recovery and Alice Zuberg's intervention overwhelm him. His Fluctlight is destroyed during this confrontation, ensuring his permanent virtual demise and neutralizing his real-world threat. His storyline concludes with his elimination as a major antagonistic force.